FIG. 25 is a circuit block diagram of conventional oscillator circuit 130 disclosed in Patent Literature 1. Oscillator circuit 130 includes oscillator 131, filter 132, and driver 133. Filter 132 receives a monitoring signal output from oscillator 131 and outputs the filtered signal to driver 133. Receiving the filtered signal, driver 133 amplifies the filtered signal and outputs a driving signal to oscillator 131. The monitoring signal contains an undesired frequency signal caused by a high-order vibration frequency of the vibration mode of oscillator 131 or caused by a vibration mode different from the driving vibration mode. Filter 132 suppresses the undesired frequency signal.
Oscillator 131 is made of crystal or other piezoelectric material with high processing accuracy, and generally, has small variations in the driving vibration frequency caused by variations in processing accuracy. However, in the case that oscillator 131 has a small size, oscillator 131 has variations in processing accuracy which greatly affect the driving vibration frequency. Oscillator 131 vibrating in plural vibration modes other than the driving vibration mode has large variations in processing accuracy, accordingly, increasing variations in the driving vibration frequency.
An oscillator made of silicon has frequency-temperature characteristics inferior to those of an oscillator made of crystal. That is, the driving vibration frequency of oscillator 131 is affected by an ambient temperature thereof.
Besides, the driving vibration frequency of oscillator 131 can be changed due to aging degradation.
In conventional oscillator circuit 130, however, the passing characteristic of filter 132 cannot optimized for variations in the driving vibration frequency caused by variations in processing accuracy of oscillator 131, changes in the driving vibration frequency caused by the ambient temperature and the aging degradation.
FIG. 26 is a circuit block diagram of conventional inertial sensor 620 disclosed in Patent Literature 2. Inertial sensor 620 includes oscillator 621, driver 622, detector circuit 623, and abnormal-state detector 624. Receiving a monitoring signal from oscillator 621, driver 622 amplifies the monitoring signal and transmits a driving signal to oscillator 621. Detecting a sensing signal output from oscillator 621, detector circuit 623 outputs the sensing signal to output terminal 625. Abnormal-state detector 624 outputs an abnormal-state detection signal to diagnostic terminal 626 based on the monitoring signal. Oscillator 621 outputs the sensing signal according to an amount of inertia applied from the outside of the oscillator. Detecting the sensing signal, detector circuit 623 outputs the sensing signal to output terminal 625. Abnormal-state detector 624 has detector section 624A and window comparator 624B. Detector 624A outputs an amplitude of the monitoring signal. Receiving the amplitude, window comparator 624B determines whether or not the amplitude is within a predetermined range between upper threshold VR501 and lower threshold VR502. If the amplitude is not less than upper threshold VR501 or not more than lower threshold VR502, window comparator 624B outputs the abnormal-state detection signal to diagnostic terminal 626. That is, if the amplitude of the monitoring signal is out of the predetermined range between upper threshold VR501 and lower threshold VR502, abnormal-state detector 624 determines that oscillator 621, i.e., inertial sensor 620, is in an abnormal state and outputs the abnormal-state detection signal to diagnostic terminal 626.
If inertial sensor 620 has a large impact from outside, the amplitude of the monitoring signal generally exceeds upper threshold VR501 temporarily, and therefore, abnormal-state detector 624 outputs the abnormal-state detection signal. If oscillator 621 stops oscillating due to braking of wire or other problems, the amplitude becomes smaller than lower threshold VR502. In this case, abnormal-state detector 624 also outputs the abnormal-state detection signal. An electronic device including inertial sensor 620 can determine that the amount of inertia which is obtained based on the sensing signal and which is output from output terminal 625 is not reliable while the abnormal-state detection signal is output from diagnostic terminal 626.
Abnormal-state detector 624 determines the abnormal state based on the amplitude of the monitoring signal. If oscillator 621 performs an abnormal oscillation, detector 624 may fail to output the abnormal-state detection signal. For example, when oscillator 621 performs an abnormal oscillation having a harmonic frequency of a driving vibration frequency or other natural resonance frequencies, oscillator 621 may maintain the vibration. In this case, the amplitude of the monitoring signal is remained constant. Besides, when the amplitude is remained within the predetermined range, no abnormal-state detection signal is output from abnormal-state detector 624. While vibrating in such abnormal oscillation state, inertial sensor 620 fails to output a proper sensing signal according to motion applied from the outside, hence degrading the reliability of the amount of inertia due to the inaccurate sensing signal output from output terminal 625.